The second feature by director, writer and actor Craig Roberts, feels like the kind of film which rarely gets made any more. Eternal Beauty is a gentle, measured drama which isn't afraid to build a vivid portrait of its characters' inner lives, while still proving unflinching in its exploration of the effects of a mental health crisis. Sally Hawkins' masterfully-formed lead, the schizophrenic Jane, is to be sympathised with, but not pitied; amid the mess those closest to her find their lives in, her fortitude gives her an uncanny power.

'It's nice that people think it's a character piece,' says Roberts, the 29-year-old star of Richard Ayoade's 2010 coming-of-age drama Submarine, who has more recently been seen in the Amazon drama Red Oaks and last year's Tolkien. 'I think in a world of Marvel movies, Scorsese would be happy with that. There are many elements which make it hard getting a film like this made, and part of that is what people are consuming now. From just the experience of this film, the mental health aspect was also hard; people were unsure as to how it would be told or how people would respond to it.'

Yet Roberts has taken a very personal subject and breathed life into it with relatable realism, while resisting the urge towards the feelgood. 'I set out with the idea of trying to recast the perceived weakness of mental health as a strength,' he says. 'Maybe it's something more than a weakness: maybe it's a power? I grew up with somebody in my family who had this superpower, and I didn't dig deep when I was younger, but about five years ago I started asking a lot more questions. I found it fascinating and wanted to share it with the world. She also lights up every room, so I wanted to stay away from the stereotypes and highlight the positives.'

The only film which Roberts said he could find which was a direct influence – in terms of centring upon a lead female character in the throes of mental illness – was John Cassavetes' A Woman Under the Influence from 1974, although Adam Sandler's anxiety-stricken lead in Paul Thomas Anderson's Punch-Drunk Love was also a reference point. In tone, the film is reminiscent of Lynne Ramsay or Mike Leigh, and the latter's influence runs across the casting. The brilliant Hawkins (who came to this on the back of her second Academy Award nomination for The Shape of Water) was the breakout lead of Leigh's Happy-Go-Lucky (2008), while David Thewlis – Jane's unpredictable lover Mike – lit up the screen in Leigh's 1993 Naked. Amid the rest of a great cast, Penelope Wilton is a hellish mother and Billie Piper plays Jane's self-obsessed sister Nicola with visceral unpleasantness.

'As soon as Sally attached herself, it made it easier because actors want to work with her,' says Roberts. 'She helped push the whole thing through. I'm glad we got to tell this story. It's just about the average person; it's not A Beautiful Mind, not about a genius who has been forced into this state of mind. It's just about a person who is living her life.'

Eternal Beauty, Glasgow Film Theatre, Wed 4 & Thu 5 Mar.